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A Handbook of Biology

Å. SÇLËRÅ

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The external layer formed of a dense connective tissue.

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Anterior transparent portion of sclera is called cornea which consists of

dense matrix of collagen and is most sensitive portion of the eye.

(NEET 2019)

B. ÇHØRØÏD

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2

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Bluish middle layer. Contains many blood vessels and appear

bluish in colour.

Choroid is thin over posterior two-thirds of the eyeball,

but it is thick in the anterior part to form ciliary body.

Ciliary body continues forward to form a pigmented

and opaque structure called the iris. It is the visible

coloured portion of the eye.

Iris has a central opening called pupil. The diameter of

the pupil is regulated by the muscle fibres of iris.

The eyeball contains a transparent crystalline lens. It is

held in place by ligaments attached to the ciliary body.

(NEET 2018)

B. RËTÏÑÅ

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Inner layer. It contains 3 layers of cells – from inner to outer ganglion

cells, bipolar cells & photoreceptor cells.

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Photoreceptor cells are 2 types: rods and cones. They contain

photosensitive proteins (photopigments).

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Photopigments are formed of opsin (a protein) and retinal (an aldehyde

of vitamin A).

(NEET 2017)

ÇØÑË ÇËLLS:

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Function: Daylight (photopic) vision & colour vision. There are 3 types of

cones containing photopigments that respond to red, green and blue

lights. The sensations of different colours are produced by combinations

of these cones and their photopigments. When the cones are stimulated

equally, a sensation of white light is produced.